This invention relates to a method of repairing the surface of a damaged metallic body, and, in particular, for repairing a damaged body of sheet metal.
Sheet metal bodies of motor vehicles are subjected to damage from many sources. Such damage can be caused by collisions with other vehicles or stationary objects, or damage can be brought about by corrosion caused by sodium chloride present in the environment or in road salt.
Many methods for repairing damaged sheet metal surfaces are known. Some years ago, it was common practice to repair a dented surface or a hole in the body by flattening out the surface as well as possible and then applying molten lead to the metal surfaces after the paint had been removed. This molten lead adhered to the metal surface and was built up to a thickness sufficient to produce a smooth outer surface. The repaired area was sanded to provide a smooth continuous surface, and the repaired area was then painted to the color of the original body.
This process of "leading" the injured areas was not only time consuming, but it also required a relatively high degree of skill to accomplish good results. It became more and more common to patch the injured areas through the use of sheets of cold rolled steel shaped to fit the injured area and marginally welded to the metal forming the body. After grinding the marginal edges of the patch and welded area, to produce a smooth surface, the area could be repainted. If the work was skillfully done, there would be little evidence of the repair.
Holes in car bodies have also been repaired by riveting or bolting panels of metal over the damaged area, grinding or mending the patch as smooth as possible, and then leading the adjoining area or applying a suitable filler before repainting. This method is acceptable if the edges of the patch are sealed against the surface repaired, but is costly and time consuming.
The welding or riveting method of repairing car bodies practically replaced the repairing of surfaces by the leading process when plastic filler compositions were developed which could be applied over the patched area to conceal the imperfections.
Such filler compositions are prepared by combining a mixture of a curable polymeric material with a catalyst hardener, applying the mixture to the metal surface to be covered, and allowing the plastic to set. The surface is then sanded and the edges feathered into the surface of the metal itself, covering the entire sheeted area. If the sanding operation is done carefully, the repair is almost impossible to see from visual inspection. This method may lead to some difficulties, particularly when the sheet is applied over a hole or rusted area of the body. Because the inner surface of the metal sheet used for the repair is exposed to the elements through the hole being covered, there is a tendency for the metal to rust out from the under surface after a period of time. Furthermore, because fillers require mixing, and because they are viscous, air becomes entrapped in them during the mixing process, creating voids upon curing, which can allow moisture to penetrate the patch, leading to corrosion.
U.S. Pat. No. 3,470,048 describes a method of patching metal surfaces, such as car bodies, with the use of a patch comprising a backing of sheet metal, such as steel, having bonded thereto a thin foam plastic sheet having a pressure-sensitive adhesive surface protected by a removable release liner. After the surface to be mended is cleaned, the release liner is removed and the patch is applied over the surface to be mended. The edges of the patch are forced into contact with surface to be patched, connecting the metal backing sheet to the surface to be patched through the foam plastic. The patch is then covered with a filler composition which is permitted to cure or dry and the exposed surface ground to the proper contour. The patch described in this patent is quite weak in peel, having a peel strength of only 4.8 lb./in. In peel, the foam would be likely to tear quite easily, which could lead to failure of the repair. Furthermore, the patch described in this patent uses a polyurethane foam of the open-celled type. Any moisture that reaches the edge of the foam will be quickly wicked through it, similar to a sponge. This wicking action will promote corrosion instead of preventing it.
U.S. Pat. No. 3,850,718 describes a metal patching plate fabricated of either aluminum or thin gauged steel which is backed with a layer of high density foam which is double coated with a pressure sensitive adhesive layer. The metal patching plate is useful for the repair of damaged truck trailers and seagoing cargo containers.
U.S. Pat. No. 4,473,419 describes a method comprising the steps of cleaning a repair area, molding a metal patch to conform to the body surface as required, peeling the coating from the adhesive, and pressing the adhesive side of the patch against the body to adhere the patch thereto.